The replication of avian sarcoma viruses (ASV) can be used as a model systems for the study of the mechanisms of recombination of DNA elements and the mechanisms which govern the regulation of transcription of viral genes. A large amount of information is available regarding the genetic and physical structure of the ASV genome, the steps by which viral RNA is converted to a DNA intermediate and the transcription of integrated viral DNA sequences. Although the integration of viral DNA into the host chromosome is an efficient event, very little information is available regarding the actual enzymatic functions required for the covalent insertion of viral DNA. Similarly, little information is available regarding the enzymatic events which lead to the transcription and possible regulation of transcription of viral genes. The objective of the experiments described in this proposal is to define the biochemical processes (DNA sequences, enzymes, and cofactors) required for the integration and transcription of the avian tumor virus genome. Viral DNA segments will be obtained by molecular cloning techniques and used to transfect avian mammalian cells. To define the viral DNA sequences required for integration and transcription of viral RNA, we will introduce deletions at specific points within the region of the viral genome required for integration. Cells transformed with viral DNA segments containing viable deletions will be analyzed to determine how the deletion of specific sequences influences integration and transcription of viral RNAs. Finally, we will attempt to establish in vitro assays for the various biochemical steps in integration. Initial attempts will be aimed at identifying proteins (of either viral or host cell origin) which specifically bind to viral DNA and/or introduce specific endonucleolytic cleavages within viral DNA.